The objectives of this investigation are to a) further define the normal development of human cellular immunity, b) define abberations of development which result from immaturity, neonatal illness, intrauterine malnutrition and postnatal malnutrition and c) investigate the mechanisms for the observed defects. Pediatric patients with increased susceptibility to infection will be studied, particularly high risk newborns with primary diseases, newborns small for gestational age as a result of intrauterine malnutrition and infants and children with acquired chronic malnutrition, including those undergoing hyperalimentation. Cellular immune studies will include quantitating peripheral blood T and B lymphocytes and monocytes, measuring lymphocyte proliferative responses, assessing lymphocyte mediator production, assessing cytotoxicity of lymphocytes, and assessing monocyte functions.